Embodiments of the present application generally relate to control strategies for lean burn spark ignition engines.
The quantity of nitrogen oxides (NOx) generated during operation of internal combustion engines, including, for example, lean burn spark engines, can be attributed, at least in part, to in-cylinder temperatures. Accordingly, certain attempts to control NOx generation have included controlling the temperature of the associated combustion event and/or the in-cylinder temperature. Other systems may attempt to control in-cylinder temperatures by controlling in-cylinder pressure and/or flame speeds.
In-cylinder temperatures and the properties of the fuel that is utilized during combustion events can also generally be correlated to the presence, and potential increases, of engine knock. Moreover, as in-cylinder temperatures increase, both the instances of engine knock and the quantity of generated NOx can also increase. While engine knock can be attributed to a variety of factors, for at least certain types of fuel used with lean burn spark engines, such as, for example, engines that utilize natural gas as a fuel, engine knock can be attributed to changes in fuel properties. Such changes in fuel properties can include, but is not limited to, a methane number (MN) of a natural gas fuel and the presence and/or quantity of diluents in fuel, such as CO2, among other diluents.
Current control strategies that attempt to reduce NOx generation in lean burn spark ignition engines often target a fixed NOx level. Such strategies can include sensing a NOx level, such as, for example, by NOx sensors, and, if necessary and based on the sensed NOx levels, adjusting the air-to-fuel mixture delivered to the cylinders in at least an attempt to adjust the NOx levels. Other approaches may also be employed that estimate the NOx level, such as, for example, a Torque Over Boost (TOB) method. Yet, such controls may not accommodate, or adjust to, changes in certain conditions, such as, for example, a change in one or more properties of the fuel being supplied for combustion in the engine. Moreover, such changes in the property(ies) of the fuel can adversely impact efforts to adjust operations of the engine system that seek to adjust NOx levels.
Further, current controls often change or adjust certain engine operations based at least in part on engine knock margin. However, the different manners in which NOx and engine knock levels have been controlled often results in conflict between the control strategies for NOx levels and the control strategies for engine knock. Moreover, the control strategies for controlling NO x levels and for controlling engine knock levels can fight each other as the strategies attempt to attain target NOx and/or engine knock levels. Such conflicts can adversely impact not only the ability to attain such target levels, but can also adversely impact operation of the internal combustion engine.